1. Technical Field
The present invention relates generally to semiconductor devices and, more specifically, to a semiconductor substrate having both bulk chip areas and areas of silicon on insulator in which selected areas of silicon on insulator are electrically connected to the wafer so as to reduce floating body problems and methods of forming the same.
2. Relevant Art
Conventional or bulk semiconductor devices are formed in semiconductive material by implanting a well of either P-type or N-type material in a wafer of either type material. Gates and source/drain diffusions are then manufactured using commonly known processes. These form devices known as metal-oxide-semiconductor (MOS) field effect transistors or FETs. When a given chip uses both P-type and N-type, it is known as a complementary metal oxide semiconductor ("CMOS"). Each of these devices must be electrically isolated from the others in order to avoid shorting of the circuits. A relatively large amount of surface area is needed for the electrical isolation of the various FETs, which is undesirable in the current trends of size reduction and greater integration. Additionally, parasitic paths and junction capacitance problems may also occur because of the source/drain diffusions' physical proximity to other FET's and the bulk substrate. These problems also result in difficulties when trying to scale down to the sizes necessary for greater integration. Furthermore, sub-threshold slope and substrate sensitivity result in difficulties in scaling bulk CMOS technology to low-voltage applications.
In order to deal with these problems, silicon on insulator ("SOI") has been gaining popularity. However, SOI suffers from the problems of self-heating, electrostatic discharge susceptibility, low breakdown voltage, and dynamic floating body effects which present problems for passgate devices and devices requiring tight threshold voltage control. Floating body effect occurs when the body of the device is not connected to a fixed potential and, therefore, the device takes on charge based on the history of the device. Specifically, in dynamic random access memory ("DRAM") the floating body effect can be especially detrimental because it is critical that the pass transistor stays in the "off" condition to prevent charge leakage from the storage capacitor. Another problem that is specific to SOI is that the formation of large value capacitors (i.e., for decoupling applications) is very difficult because a specific purpose of SOI is to reduce junction capacitance. Additionally, the thin layer of semiconductor makes it difficult to create low resistance discharge paths for electrostatic discharge ("ESD") devices.
Because of these drawbacks, it has been suggested that the best scenario would be to combine areas of SOI for high performance support devices, with adjacent bulk devices for low leakage memory arrays. However, forming both the SOI areas and the bulk areas is difficult at best.